Processing seismic data

ABSTRACT

A method of processing seismic data comprises identifying the value of a first parameter associated with an event in a first set of seismic data. The value of a second parameter associated with a corresponding event in a second set of seismic data is then obtained using at least one look-up table. The first parameter may be, for example, PP travel time with the first data set being a raw PP data set, and the second parameter may be, for example, PS travel time with the second data set being a raw PS data set or reflection depth. The invention makes it possible to identify pairs of corresponding PP and PS events in raw data traces. The look-up table(s) are obtained using an assumed model for the velocity of propagation of acoustic energy within the earth. The results of the method may be used in quality control, or may be used to correct the velocity model.

The present invention relates to processing seismic data, in particularto processing multi-component seismic data. In particular, it relates toprocessing multi-component seismic to determine an event in one datacomponent that corresponds to an event in another data component.

FIG. 1 is a schematic illustration of a seismic survey. As is wellknown, a seismic survey is performed using at least one seismic source 1and an array of seismic receivers 2, 3, 4. In FIG. 1 the source 1 andthe receivers 2, 3, 4 are shown disposed on the earth's surface, butother arrangements are known such as, for example, disposing receiversin a borehole. When the source 1 is actuated, acoustic energy is emitteddownwards into the earth, and is reflected by geological structureswithin the earth. The reflected energy is detected at the receivers 2,3, 4.

FIG. 1 shows two geological structures 5, 6 that act as partialreflectors of acoustic energy. These structures may, for example, beinterfaces between two layers of the earth's interior that havedifferent acoustic properties. As a result, the data acquired at each ofthe receivers 2, 3, 4 contains one “event” corresponding to partialreflection of acoustic energy at the upper interface 5 and another“event” corresponding to partial reflection of acoustic energy at thelower interface 6. When seismic data are processed, each interfaceresponsible for reflection of seismic data is allotted a unique“interface index”, and events may be classified according to the indexof the interface responsible for the event.

Typical traces that would be acquired by the receivers 2, 3, 4 of FIG. 1are shown in FIG. 2. The vertical axis of FIG. 2 denotes time since theactuation of the seismic source. The traces are arranged in order ofincreasing source-receiver distance (known as offset). Thus, the tracehaving the lowest offset is at the left, and offset increases to theright. Within each trace, the horizontal scale provides a measure of theamplitude of seismic energy acquired at each receiver. “A2”, “A3” and“A4” denote the amplitude of the trace acquired at the receiver 2, atthe receiver 3 and at the receiver 4 respectively.

If the three traces in FIG. 2 are compared, it will be seen that eachtrace contains an event B corresponding to reflection at the upperinterface 5 and another event C corresponding to reflection of acousticenergy at the lower interface 6. However, events corresponding toreflection at a particular interface do not occur at the same time ineach trace. For example, the event B occurs at time t₁ in the traceacquired at receiver 2, but occurs at greater times in the tracesacquired by other receivers. This is because the overall path lengthfrom the source to the receiver, and hence the travel time, increasewith increasing source-receiver separation. The increase in arrival timeof an event with increasing offset is known as “moveout”.

Many seismic surveys use multi-component seismic receivers, which areable to acquire at least two components of the seismic wave fieldincident on the receiver. A 3-component, or 3-C, receiver, for example,records three orthogonal components of the seismic wavefield, and theseare normally taken to be the x-, y- and z-(vertical) components of thewavefield.

Acoustic energy emitted by the seismic source 1 is predominantly apressure-wave (or p-wave). When the energy undergoes reflection aninterface 5, 6, however, it may also undergo partial mode conversion toa shear wave (s-wave). The seismic wavefield acquired at the receiver 2,3, 4 will therefore both contain p-waves and s-waves. Events arisingfrom arrival of p-waves are generally referred to as PP events, sincethey involve seismic energy that is emitted as a p-wave and that isincident on the receiver as a p-wave. Events arising from s-waves aregenerally referred to as PS events, since they arise from acousticenergy which is emitted as a p-wave and which undergoes mode-conversionto an s-wave upon reflection and so is incident on the receiver as ans-wave. PP events occur most strongly in vertical components of theacquired seismic data, whereas PS events appear most strongly in thehorizontal component of the acquired seismic data

Where partial mode conversion occurs, the seismic energy reflected as ap-wave gives rise to a PP event in the acquired seismic data and theseismic energy reflected as a (mode-converted) s-wave gives rise to acorresponding PS event in the acquired seismic data. A PP event and a PSevent are said to be “corresponding events” if the PP event and the PSevent involve reflection at the same interface within the earth'sinterior. The velocity of propagation of a p-wave through the earth isgenerally not equal to the velocity of propagation of an s-wave, so thata PP event in seismic data acquired at a receiver will in general notoccur at the same time as the corresponding PS event. Whenmulti-component seismic data is processed, it is often desirable toidentify corresponding pairs of a PP event in the vertical component ofthe seismic data and a PS event in a horizontal component of the seismicdata. This allows information about the reflector to be obtained fromthe PP data and from the PS data

The data traces shown in FIG. 2 represent seismic data tracesessentially as acquired at the receiver 2, 3, 4. These are generallyreferred to as “raw” data traces.

In conventional seismic data processing, the raw data traces of FIG. 2are first processed to compensate for the source-receiver offset. Theeffect of this processing is to transfer each event in a trace to thetime at which it would have occurred if there had been zerosource-receiver offset—i.e. if the source and receiver were co-incident.If the correction for offset is performed correctly, an eventcorresponding to reflection at one interface should occur at the sametime in each offset-corrected trace—the event should have zero moveoutin the corrected traces, and correction to zero-offset is therefore knowas “moveout correction”. The traces corrected to zero offset may then beaveraged, or “stacked”, and this attenuates random noise in the traces.

Methods have been proposed for identifying corresponding PP and PSevents in stacked seismic data. In general, these methods assume thatthere is a constant linear relation between the arrival time of a PPevent and the arrival time of the corresponding PS event. The arrivaltime of an event in the PP data can be mapped onto the expected arrivaltime of the corresponding event in the PS data by multiplying the PParrival time by a constant factor, known generally as “vertical gamma”.The “vertical gamma” factor is essentially a squeeze/stretch factor,that stretches or squeezes the vertical axis (time axis) of traces for avertical component of the seismic data to have the same scale as thevertical axis (time axis) of traces of a horizontal component of theseismic data.

The magnitude of the “vertical gamma” factor may be determined simply bymanual identification of pairs of corresponding PP and PS events in thestacked seismic data, and deriving the vertical gamma factor from theirrespective arrival times. It is also known to use an interactiveapproach in which an initial value of the vertical gamma factor ispicked from the stacked traces for the horizontal and verticalcomponents, and is then used to assist in identification of furtherpairs of corresponding PP and PS events. Once further pairs ofcorresponding events have been identified, their arrival times may beused to refine the value of the vertical gamma factor.

These prior art techniques may not, however, be applied to raw datatraces of the type shown in FIG. 2. The source-receiver offset variesfrom one raw trace to another, so that the arrival time of an eventdepends on the offset as well as on the velocity of propagation ofacoustic energy. It is therefore not possible to match the arrival timesof events in raw horizontal data traces with arrival times of events inraw vertical data traces using a constant scaling factor.

The present invention provides a method of processing seismic data, themethod comprising:

a) identifying the value of a first parameter associated with an eventin a first set of seismic data; and

b) obtaining, using at least one look-up table, the value of a secondparameter, the second parameter being associated with a correspondingevent in a second set of seismic data.

The look-up table or look-up tables may give values for parameters ofthe sets of seismic data, such as the PP travel time, the PS traveltime, or the depth at which a reflection occurs, in terms of parametersof the survey such as the source-receiver offset and the index of theinterface at which reflection occurs. The look-up tables thereforeimplicitly include velocity information, since they are derived using,for example, a particular model for the velocity of propagation ofacoustic energy in for the earth's interior. The method of the presentinvention may therefore be applied to raw data traces, and allows, forexample, an event in a raw PS data set (that is, an event in a set ofraw PS data traces) that corresponds to an event in a raw PP data set.

The invention also provides an apparatus for processing seismic data,comprising: means for identifying the value of a first parameterassociated with an event in a set of seismic data; and means obtaining,using first and second look-up tables, the value of a second parameter,the second parameter being associated with another event in the set ofseismic data.

Preferred features of the present invention are defined in the dependentclaims.

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying figuresin which:

FIG. 1 is a schematic illustration of a land-based seismic survey;

FIG. 2 shows typical data traces acquired in the seismic survey of FIG.1;

FIG. 3 illustrates a method according to a first embodiment of thepresent invention;

FIG. 4 illustrates a method according to a second embodiment of thepresent invention;

FIG. 5 is a schematic illustration of a third embodiment of the presentinvention;

FIG. 6 illustrates a method of generating a look-up table; and

FIG. 7 is a schematic block diagram of an apparatus according to thepresent invention.

Preferred embodiments of the present invention will now be described indetail, by way of illustrative example.

The principle of the present invention is to generate two or morelook-up tables that give values for respective parameters of theacquired seismic data as a function of 1 or more other parameters. Inthe embodiment described below, three look-up tables are used, but theinvention is not limited to this number of look-up tables. In theembodiment described below the three look-up tables relate to,respectively, the reflection depth of an event, the PP travel time of anevent, and the PS travel time of an event, and are a function of thesource-receiver offset and the index of the interface. The term“reflection depth” of a seismic event, as used herein, denotes the depthat which the reflection of energy which gives rise to that event occurs.

In general, a look-up table for a parameter P has the form of: “LUT P(source position, receiver position, interface index)”. In the case of aone-dimensional seismic surveying arrangement, in which the source(s)and receiver(s) are arranged along a straight line, the source positionand receiver position can be replaced by a single parameter indicativeof the distance between the source and receiver, such as offset. Thus,in a 1-D case, a look-up table has the general form LUT P (offset,interface index). In a 2-D or 3-D case, however, the co-ordinates (x,z)or (x,y,z) of the source and the receiver need to be taken account of.The embodiment of the invention described below refers to the 1-D casefor simplicity, but the invention may be applied in a 2-D or 3-D case.

In this embodiment, therefore, the three look-up tables are as follows:

LUT time PP (offset, interface index);

LUT time PS (offset, interface index); and

LUT depth (interface index).

(The interface depth is not a function of offset, so the depth LUTdepends only on the interface index.)

FIG. 3 illustrates one method according to the present invention. Of thefour panels in FIG. 3, the left-hand panel D shows raw data traces forthe vertical component of acquired seismic data—and these will bereferred to as PP raw data traces since, as noted above, the verticalcomponent of the acquired seismic data contains predominantly PP events.

The next panel E in FIG. 3 corresponds generally to panel D, but showsraw data traces for a horizontal component of acquired seismic data.These traces contain predominantly PS events, and so are referred to asPS raw data traces.

Panel F in FIG. 3 contains offset-corrected traces corresponding to theraw data traces in panel D. That is, each trace in panel F is obtainedfrom the corresponding trace in panel D by correcting the trace to zerooffset. Furthermore, a trace that is corrected to zero offsetcorresponds to a seismic energy path that penetrates vertically into theearth to a certain depth, so that there is a direct relationship betweenthe travel time of seismic energy and the depth at which the seismicenergy is reflected. The vertical axis of the traces in panel F hastherefore been converted from time to depth, whereas the vertical axesin panels D and E represent time. The correction to zero-offset, and thetime-to-depth conversion, are carried out using an assumed model for thevelocity of propagation of p-waves and s-waves. In FIG. 3, the velocitymodel has been obtained using ray-tracing, as will be described below.However, the correction to zero offset and the time-to-depth conversionmay alternatively be carried out using a velocity model in which thep-wave velocity is assumed to be approximated by an “effective velocity”which is constant and isotropic (correcting for offset using this modelis the well-known “normal moveout” or NMO correction).

It will be seen that whereas the events in the raw PP data traces ofpanel D occur at different times in traces acquired at differentsource-receiver offsets, most events in the offset-corrected PP datatraces of panel F occur at the same time in each trace.

Finally, panel G (the right-hand panel) shows the PS data traces ofpanel F after correction to zero source-receiver offset andtime-to-depth conversion. The correction to zero-offset and thetime-to-depth conversion for ps-waves are carried out using the samemodel for the velocity of propagation of p-waves and s-waves within theearth. In the embodiments of FIG. 3 ray tracing was again used, althoughan “effective velocity” model may alternatively be used for s-waves (butthe velocity used in the NMO correction for PS events will, in general,not be the same as the velocity used in the NMO correction for PPevents).

In the traces shown in FIG. 3, the traces are arranged in the order ofincreasing source-receiver offset, and within each trace the horizontalscale is indicative of the amplitude of the seismic energy incident onthe receiver. In panels D and E the vertical axis represents time sinceactuation of the source. For the offset-corrected traces in panels F andG, the seismic energy path is now a path that simply penetratesvertically into the earth to a certain depth, and there is a directrelation between the zero-offset travel time and the reflection depth.While the vertical axis in panels F and G could represent zero-offsettravel time, it may be more useful if the vertical axis in panels F andG is converted, as shown in FIG. 3, to represent the reflection depth asexplained above. FIG. 3 thus shows four sets of data—a raw PP data set,a raw PS data set, an offset corrected (and time-to-depth converted) PPdata set and an offset corrected (and time-to-depth converted) PS dataset

In the method illustrated in FIG. 3, an event in one of the data sets isinitially selected. This is represented by step 1 in FIG. 3. In FIG. 3,the selected event is in the raw PP data set, being an event in one ofthe PP raw data traces of panel D, but the trace that is selectedinitially could equally well be in one of the PS raw data traces.

Selecting an event in a particular data set, by selecting an event in araw data trace, defines a time, corresponding to the arrival time of theevent. Where the selected event is a PP event, a PP travel time isdefined. Selecting an event also defines an offset, corresponding to thesource-receiver offset used to acquire the trace in which the selectedevent occurs. For the particular PP event selected in FIG. 3, the offsetis 3003 metres and the PP travel time is 2.131 seconds.

Since both the PP travel time and the offset are known, the “LUT timePP” look-up table may be used to determine the interface indexcorresponding to the selected event.

Next, at step 2, the “LUT time PS” look-up table is used to find the PStravel time corresponding to the offset (as determined for the originalselected event) and the interface (as determined from the “LUT time PP”look-up table). This produces the PS travel time of the PS event thatcorresponds to the PP event selected at step 1. In FIG. 3, the PS traveltime corresponding to the selected PP event is determined to be 3.611seconds.

Once the PS travel time corresponding to the selected PP event has beendetermined, it is straightforward to identify, in the raw PS data set,the PS event corresponding to the selected PP event. The correspondingPS event is the event in the raw PS data that occurs in the raw PS datatrace having the same offset as the selected PP event and that occurs atthe PS travel time determined in step 2. This is indicated as step 3 inFIG. 3.

The present invention thus allows corresponding PP and PS events to bedetermined in raw data traces.

The depth of the reflection point that gives rise to the event selectedat step 1 may also be determined, using the “LUT depth” look-up table.In FIG. 3, the reflection depth corresponding to the selected PP eventis determined to be 2073.291 metres.

In a preferred embodiment, the traces are displayed as shown in FIG. 3on a computer screen. The event selected at step 1 is selected by, forexample, positioning the cursor of a computer mouse over the selectedevent and “clicking” the mouse button. When the corresponding PS traveltime has been determined, the event that occurs at that travel time inthe raw PS data trace having the same offset at the selected event maybe automatically highlighted on the computer screen in some way. Thismay be done, for example, by changing the background colour of a smallregion of the screen surrounding the corresponding PS event.

Additionally or alternatively, it is possible to identify an event inthe offset-corrected PP and/or PS data sets that has the same offset asthe originally selected event and that has the depth determined from the“LUT depth” look-up table. This is indicated in FIG. 3, as step 4. Theevent identified in the offset-corrected PP and/or PS data sets mayagain be highlighted on the computer display in any convenient manner.

The above is a simple description of this embodiment, and assumed thatwhen the interface index of the initially-selected event was determined,there would be an exact match. This may not always be the case, and in amore realistic embodiment an interpolation process is used. In thismethod, the following detailed steps are involved. It will be assumedthat this embodiment is implemented by the user moving a cursor throughthe raw PP data set and selecting an event using the cursor. However,the invention is not limited to this.

The cursor location when the user selects an event in panel D gives thePP travel time tpp and the source-receiver offset o of the selectedevent. The model based cursor tracking algorithm then finds interfaceindex i such that:LUTtimePP(o,i)<tpp<LUTtimePP(o,i+1).

It is then possible to compute a linear interpolation factor α whereα=(LUTtimePP(o,i+1)−tpp)/(LUTtimePP(o,i+1)−LUTtimePP(o,i)

Thus, it is possible to find the corresponding PS time tps and depth ofthe reflection point d with a linear interpolation of the correspondinglook-up tables:tps=aLUTtimePS(o,i)+(1−α)LUTtimePS(o,i+1)d=αLUTdepth(i)+(1−α)LUTdepth(i+1)

The values determined for tps and d may be displayed to the user, asshown in the bottom right of FIG. 3. Additionally or alternatively, thedisplay may highlight the corresponding point at (o, tps) in the raw PSseismic data set of panel E and/or the corresponding points at (o, d) inthe offset-corrected and time-to-depth corrected PP and PS seismic datasets of panels F and G.

In the examples described above the user has initially selected an eventin the raw PP data set, for example by placing the cursor of a computermouse over the event in the displayed image. The user couldalternatively select an event in the raw PS data set of panel E, and inthis case the method of the invention would provide the corresponding PPtravel time and/or the corresponding depth. As a further alternative,the user could select an event in the offset-corrected PP or PS datasets of panel F or G, and in this embodiment the method would providethe PP and PS travel times of the corresponding events in the raw PPdata set and the raw PS data set.

FIG. 4 illustrates a further embodiment of the invention. The four setsof data shown in FIG. 4 are again PP raw data traces (panel D), PS rawdata traces (panel E), PP offset-corrected and time-to-depth converteddata traces (panel F) and PS offset-corrected and time-to-depthconverted data traces (panel G). The traces are shown in FIG. 4 with ahigher enlargement than in FIG. 3, so that correspondingly fewer tracesare shown, over a shorter time window.

In this embodiment, it is assumed that the user implicitly selects anevent in an offset-corrected data trace, by moving scroll bars of panelF or panel G to position a selected event near the centre of the“window” defined by the scroll bars. In FIG. 4, the selected event isshown as an event in a PP offset-corrected and time-to-depth convertedtrace. The following detailed steps are involved when the user scrollshorizontally in the offset-corrected PP data (as denoted by the arrowlabelled 1):

The centre of the window for panel F defines an event in theoffset-corrected PP data having a reflection depth d and source-receiveroffset of o. This embodiment is implemented by a model-based scrollingalgorithm which then finds an interface having interface index i suchthat:LUTdepth(i)<d<LUTdepth(i+1)

It is again possible to compute the linear interpolation factor α whereα=(LUTdepth(i+1)−d)/(LUTdepth(i+1)−LUTdepth(i))

Thus, it is possible to find the corresponding PP time tpp and PS timetps with a linear interpolation of the corresponding look-up tables:tps=aLUTtimePS(o,i)+(1−α)LUTtimePS(o,i+1)tpp=αLUTtimePP(o,i)+(1−α)LUTtimePP(o,i+1)

The model based scrolling algorithm then scrolls the window throughwhich panel D (raw PP traces) is displayed so that its centre is (o,tpp) and scrolls the window through which panel E (raw PS traces) isdisplayed so that its centre is (o, tps). The algorithm also scrolls thewindow through which panel G is displayed so that its centre is (d, o).The windows through which the four data sets are displayed are alllinked together, so that the corresponding events are positioned at thecentre of each window.

In FIG. 4, the arrow 1 in panel F denotes horizontal scrolling throughpanel F. The arrows 3 in panels D, E and G shows the scrolling directionin these panels produced by the scrolling algorithm as a result ofhorizontal scrolling through panel F. As expected, horizontal scrollingthrough panel F also produces horizontal scrolling through panel G,since both panels shows offset-corrected data sets. However, horizontalscrolling through panel F does not produce horizontal scrolling in theraw data sets of panels D and E, since time increases with offset inthese raw data sets.

Panels F and G of FIGS. 3 and 4 show, as noted, PP traces and PS tracesthat have been corrected for offset and time-to-depth converted. Theseoffset-corrected traces are obtained using the same velocity model asused to derive the look-up tables. This is illustrated in FIG. 5 of theapplication.

To determine the offset-corrected PP traces, for example, the rawvertical component traces, the “LUTtimePP” look-up table and the“LUTdepth” look-up table are required. The two look-up tables are usedto correct the raw vertical component data traces to zero offset. Thismay be done by determining the interface index for a PP event in a tracewith a particular offset, using the “LUTtimePP” look-up table. The“LUTtimePP” look-up table may then again be used to determine the traveltime for zero offset for that interface index.

The effect of correcting raw data traces for the source-receiver offsetis to produce the trace that would have been obtained if the source andreceiver were co-incident. This would again be a trace that had timealong its vertical axis, and the amplitude of the acquired seismiccomponent along the horizontal axis. However, since the seismic energypath is now a path that simply penetrates vertically into the earth to acertain depth, there is a direct relation between the zero-offset traveltime and the reflection depth. The relationship will depend on thevelocity model used to perform the correction for offset—in the simplestcase, in which the velocity is assumed to be uniform and isotropicwithin the earth, the zero-offset travel time is proportional to thereflection depth.

In FIG. 5, therefore, once the correction for offset has been carriedout to obtain a zero-offset trace, the resultant zero-offset trace isthen depth-converted using the “LUTdepth” look-up table. This producesoffset corrected, time-to-depth corrected data, so that the verticalaxis of the trace is converted from travel time to reflection depth. Theresult, as shown in the two right-hand panels in FIGS. 3 and 4, is aseries of traces that simulate traces acquired at zero source-receiveroffset, and that have reflection depth as the vertical axis. If theoffset-corrected data is not time-to-depth converted, but is indexed bytime (i.e., displayed with time as the vertical axis) then use of theLUTtimePP look-up table is sufficient and the LUTdepth look-up table isnot required.

(It should be noted that offset corrected, time-to-depth corrected datamay be produced in one step. The steps of (1) correction for offset and(2) time-to-depth conversion have been described separately above forease of understanding, but the steps may be combined by using theLUTdepth look-up table directly instead of the LUTdepthPP look-uptable.)

The raw horizontal seismic data traces (the PS raw data traces) may becorrected for offset and, if desired, converted to show depth alongtheir vertical axes, in an analogous way as also shown in FIG. 5, usingthe LUTtimePS look-up table instead of the LUTtimePP look-up table.

In the embodiment of FIG. 5, the correction to zero-offset is carriedout using a model-based NMO correction, based on the results ofray-tracing. The invention is not limited to this specific method ofcorrecting to zero-offset, however, any suitable method may be used suchas, for example, the well-known “effective velocity” model.

As mentioned above, the look-up tables relate reflection depth, PPtravel time or PS travel time to the offset and the interface indexaccording to a chosen velocity model. Two examples of ways in which thelook-up tables may be determined will now be described with reference toFIG. 6.

One method of determining the look-up tables makes use of an effectivevelocity model in which the parameters are normal move-out velocityfields for the PP and PS waves, and a vertical gamma field. Initially,the Vnmo(PP) velocity field is converted to an interval velocity field,Vint(PP), which is indexed in depth. This conversion may be carried outusing the Dix formula. The interval velocity field may then be used tomap zero-offset PP travel time to depth, and vice versa.

The vertical gamma field is used to map from zero-offset PS travel timeto zero-offset PP time. The combination of the vertical gamma field andthe interval velocity field indirectly provide a map from thezero-offset PS travel time to depth, and vice versa.

Next, for every pair of values for source-receiver offset and interfaceindex, the zero-offset PP travel time is determined using the Vnmo(PP)velocity field; similarly, the zero-offset PS travel time is determinedfor every pair of source-receiver offset and interface index using theVnmo(PS) velocity field. The LUTdepth look-up table is also calculated,and this provides an indirect mapping from zero-offset time to depth,through the interface index.

The above method is shown as the left-hand part of FIG. 6.

An alternative method of determining the look-up tables is to useanisotropic ray tracing. In this method, an elastic, anisotropic modelof the relationship between velocity and depth is set up. Two-point raytracing from the source to the receiver is then used, to determine thetravel time associated with one trace—for example, for a PP trace thatinvolves reflection from a particular interface defined in the velocitymodel. This process is repeated for every source-receiver combination,for every interface, for both PP and PS reflections. For each trace,this method determines the travel time, which will be a PP travel timeor a PS travel time as appropriate, and the reflection depth. Thus, thismethod determines the depth, PP time and PS time for everysource-receiver offset and for every interface in the velocity modelused. These travel times and reflection depths are stored in therespective look-up tables.

Look-up tables for a 2-D or 3-D embodiment of the invention may also beobtained using ray-tracing.

The invention, as described above, provides, starting from an initialvelocity model, a method of navigating from an event in a data tracerelating to one component of seismic data to a corresponding event in adata trace relating to another component of the seismic data Theinvention may be used to provide a quality control (QC) measure. Ifthere is a discrepancy between, for example, the estimated PS traveltime for an event corresponding to a PP event and the actual position ofthe corresponding PS event, this suggests that the velocity model isincorrect. The invention may therefore be used to provide QC on thevelocity model. For QC purposes, it is sufficient to display theresults. For example the offset and PS travel time calculated tocorrespond to a selected event in the raw PP data set may be displayedon a display screen (for example the display screen of a computer) or ashard copy, as shown in the right hand lower corner of FIG. 3. Anoperator can determine whether the raw PS data set contains an event atthat time and offset.

In addition to QC purposes, the invention may also be used to refine oneor more of the look-up tables by refining the velocity model used todetermine the look-up tables. If the results show a discrepancy betweenthe estimated position of an event and the actual position of an eventit is possible to use this result to up-date the velocity model used todetermine the look-up tables, and re-calculate one or more of thelook-up tables using the up-dated velocity model. The present inventionthus makes possible dynamic, interactive correction of a velocity model.The correction may be performed in an iterative manner, until theestimated position of an event and the actual position of that event aresubstantially the same. The correction of the velocity model may beperformed as a continuous process while the user is selecting events,and the look-up tables can be recalculated “on-the-fly”.

Once corresponding PP and PS events have been determined it is much moreeasy to identify discrepancies in the velocity model. For example,residual moveout at far offsets suggests anisotropy, mis-tie betweenwell data and PP data suggest anisotropy, and mis-tie between PP and PSseismic suggest errors in the “vertical gamma” used. Assuming sufficientdata quality, a user should be able to pick interactively and interpretthe PP and the PS moveout curve for any interface, and invert for theoptimal elastic properties (velocities) for the interface.

It is generally recognized that an elastic attenuation will impose afrequency-dependent phase shift (and attenuation) to a waveletpropagating through the earth. Thus, in the case where the recorded PPand PS seismic waveforms are not perfectly in-phase with one another, itis reasonable to expect a certain mis-tie between PP and PS events,owing to the phase-shift. If an estimate of Qp(f,z) and Qs(f,z) isavailable (e.g. derived from walkaway-VSP data) so that the anelasticattenuation for each layer in the earth model is known, it is in theorypossible to apply a time-variant inverse-Q filter on the PP and PSseismic data In order to do it is necessary to use the previouslygenerated map functions to calculate the offset begin and end times foreach layer for each trace, and apply the inverse Q filter for that partof the trace. (The begin time for a layer is the time at which seismicenergy enters a layer, from above or from below, and the end time is thetime at which the seismic energy leaves the layer.) This will shift theapparent events to a more true temporal location, enabling more accurateevent matching and/or velocity inversion.

There are several ways to estimate geometrical spreading, based only ona background velocity model. A user could apply geometrical spreadingcorrection to the seismic as well, in a similar way as for the inverse-Qfilter (i.e. to calculate a time-offset variant gain, based on the mapfunctions) and apply this to the data in real-time, and re-calculate thegain function every time the velocity function is updated (e.g. byinversion of PP and PS moveout curves).

In order to model the transmission loss, an estimate of the density (inaddition to the velocity), as a function of depth, is required. Thedensity can be determined from for example RHOB (density) logs (usuallyindexed in depth). Alternatively, an empirical relationship (e.g.Gardener's equation) can be used to estimate density as a function ofvelocity and depth. The PP and PS acoustic impedance can then becalculated, and the PP and PS reflection coefficients at any offset anddepth can then be estimated by some approximation of the Zoeppritzequation, or modelled through ray-tracing. A user will thus be able tocorrect the PP and PS seismic data for transmission loss, based on thebackground velocity model and the calculated map functions, and applythe correction to the seismic data in real-time.

The incidence angle for the propagating wave at any interface for anoffset can always be estimated as well. If the map functions aregenerated by ray-tracing, an exact estimate of the incidence angle isalways calculated, and this could be recorded for later use in separatelook-up tables. This angle information can be used to sort the PP and PSseismic gathers into angle bands, or the incidence-angles as a functionof depth and offset could be plotted as contour lines on top of the PPand PS seismic gathers in real time. Optionally, the user will then beable to link the horizontal movement of the cursor and scroll bar to theincidence angle field, enabling a horizontal cursor tracking based onincidence angle instead of offset. For example if the user locates thecursor on a sample at interface index 5 and offset=x_(pp), in the PPseismic gather, it is possible to look up the incidence angle α_(pp) forthe propagating wave at that location. The corresponding offset x_(ps)in the PS gather which has the same incidence angle for the giveninterface can then be determined, and that location in the PS gather canbe highlighted.

The invention does have further applications. For example, since themapping functions used in the determination of the look-up tables relateto PP travel time and PS travel time at non-zero offset with the PP andPS travel time at zero-offset, and thus with the reflection depth, thelook-up tables of the invention may also be used to perform automaticoffset correction of raw PP or PS data traces. Furthermore, onceoffset-correction has been carried out, the invention may further beused to convert the vertical axis of the zero-offset traces from time todepth.

FIG. 5 is a schematic block diagram of a programmable apparatus 5according to the present invention. The apparatus comprises aprogrammable data process 6 with a programme memory 7, for instance inthe form of a read-only memory (ROM), storing a programme forcontrolling the data processor 6 to perform any of the processingmethods described above. The apparatus further comprises non-volatileread/write memory 8 for storing, for example, any data which must beretained in the absence of power supply. A “working” or scratch padmemory for the data processor is provided by a random access memory(RAM) 9. An input interface 10 is provided, for instance for receivingcommands and data. An output interface 11 is provided, for instance fordisplaying information relating to the progress and result of themethod. Seismic data for processing may be supplied via the inputinterface 9, or may alternatively be retrieved from a machine-readabledata store 12.

The output interface 11 may preferably comprise a display screen able todisplay seismic data in the manner shown generally in FIG. 3 or FIG. 4.

The programme for operating the system and for performing the methoddescribed hereinbefore is stored in the programme memory 7, which may beembodied as a semi-conductor memory, for instance of the well-known ROMtype. However, the programme may be stored in any other suitable storagemedium, such as magnetic data carrier 7 a, such as a “floppy disk” orCD-ROM 7 b.

1. A method of processing seismic data, the method comprising: a)identifying the value of a first parameter associated with an event in afirst set of seismic data; b) obtaining, using at least one look-uptable, the value of a second parameter, the second parameter beingassociated with a corresponding event in a second set of seismic data.2. A method as claimed in claim 1 and comprising obtaining the value ofthe second parameter using a first look-up table of the first parameteragainst at least one survey parameter and a second look-up table of thesecond parameter against the at least one survey parameter.
 3. A methodas claimed in claim 2 wherein step (b) comprises: b1) obtaining, usingthe first look-up table, the value of the survey parameter, or arespective value of each survey parameter, corresponding to the value ofthe first parameter associated with the event in the first set ofseismic data; and b2) obtaining, using the second look-up table, thevalue of the second parameter corresponding to the value of the surveyparameter, or the respective values of each survey parameter, determinedin step (b1).
 4. A method as claimed in claim 3 and further comprisingdefining a third look-up table of a third parameter against the at leastone survey parameter.
 5. A method as claimed in claim 4 and furthercomprising obtaining, using the third look-up table, the value of thethird parameter corresponding to the value of the survey parameter, orthe respective values of each survey parameter, determined in step (b1).6. A method as claimed in claim 2, wherein the at least one surveyparameter comprises offset and interface index.
 7. A method as claimedin claim 1 wherein the first parameter is PP travel time.
 8. A method asclaimed in claim 7 wherein the second parameter is PS travel time.
 9. Amethod as claimed in claim 4, wherein the first parameter is PP traveltime and the third parameter comprises reflection depth.
 10. A method asclaimed in claim 1 wherein the first parameter of the seismic data isreflection depth.
 11. A method as claimed in claim 1 and comprisingdisplaying the obtained value of the second parameter.
 12. A method asclaimed in claim 5, and comprising displaying the obtained value of thethird parameter.
 13. A method as claimed in claim 11 wherein thedisplaying step comprises highlighting a portion of a displayed seismictrace.
 14. A method as claimed in claim 1 and comprising modifying theat least one look-up table, on the basis of the obtained value of thesecond parameter.
 15. A method as claimed in claim 5 and comprisingmodifying the at least one look-up table, on the basis of the obtainedvalue of the third parameter.
 16. A method as claimed in claim 14,wherein the step of modifying the at least one look-up table, comprisesmodifying a model for the velocity of propagation of acoustic energywithin the earth.
 17. A method of processing seismic data comprising:determining a first look-up table of a first parameter of seismic dataagainst at least one survey parameter; and determining a second look-uptable of a second parameter of seismic data against the at least onesurvey parameter; wherein the method comprises using a predeterminedmodel for the velocity of propagation of seismic energy within the earthin the determination of the first and second look-up tables.
 18. Anapparatus for processing seismic data, comprising: means for identifyingthe value of a first parameter associated with an event in a set ofseismic data; and means obtaining, using first and second look-uptables, the value of a second parameter, the second parameter beingassociated with another event in the set of seismic data.
 19. Anapparatus as claimed in claim 18 and comprising a programmable dataprocessor.
 20. The apparatus as claimed in claim 19, wherein the firstparameter-identifying means and the second parameter-identifying meansare part of a program fixed in a storage medium the program beingexecutable by the data processor.
 21. The method as claimed in claim 1,wherein steps (a) and (b) are part of a program fixed in a storagemedium, the program being executable by a programmable data processor.22. The method of claim 1, wherein steps (a) and (b) are part of aprogram for controlling a computer.